NOx emissions from diesel engines are an environmental problem. Several countries, including the United States, have long had regulations pending that will limit NOx emissions from trucks and other diesel-powered vehicles. Manufacturers and researchers have put considerable effort toward meeting those regulations.
In gasoline powered vehicles that use stoichiometric fuel-air mixtures, three-way catalysts have been shown to control NOx emissions. In diesel-powered vehicles, which use compression ignition, the exhaust is generally too oxygen-rich for three-way catalysts to be effective.
Several solutions have been proposed for controlling NOx emissions from diesel-powered vehicles. One set of approaches focuses on the engine. Techniques such as exhaust gas recirculation and partially homogenizing fuel-air mixtures are helpful, but these techniques alone will not eliminate NOx emissions. Another set of approaches remove NOx from the vehicle exhaust. These include the use of lean-burn NOx catalysts, selective catalytic reduction (SCR) catalysts, and lean NOx traps (LNTs).
Lean-burn NOx catalysts promote the reduction of NOx under oxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere is difficult. It has proven challenging to find a lean-burn NOx catalyst that has the required activity, durability, and operating temperature range. A reductant such as diesel fuel must be steadily supplied to the exhaust for lean NOx reduction, introducing a fuel economy penalty of 3% or more. Currently, peak NOx conversion efficiencies for lean-burn NOx catalysts are unacceptably low.
SCR generally refers to selective catalytic reduction of NOx by ammonia. The reaction takes place even in an oxidizing environment. The NOx can be temporarily stored in an adsorbent or ammonia can be fed continuously into the exhaust. SCR can achieve high levels of NOx reduction, but there is a disadvantage in the lack of infrastructure for distributing ammonia or a suitable precursor. Another concern relates to the possible release of ammonia into the environment.
To clarify the state of a sometime ambiguous nomenclature, it should be noted that in the exhaust aftertreatment art, the terms “SCR catalyst” and “lean NOx catalyst” are occasionally used interchangeably. Where the term “SCR” is used to refer just to ammonia-SCR, as it often is, SCR is a special case of lean NOx catalysis. Commonly when both types of catalysts are discussed in one reference, SCR is used with reference to ammonia-SCR and lean NOx catalysis is used with reference to SCR with reductants other than ammonia, such as SCR with hydrocarbons.
LNTs are devices that adsorb NOx under lean exhaust conditions and reduce and release the adsorbed NOx under rich conditions. A LNT generally includes a NOx adsorbent and a catalyst. The adsorbent is typically an alkaline earth compound, such as BaCO3 and the catalyst is typically a combination of precious metals including Pt and Rh. In lean exhaust, the platinum catalyst speeds oxidizing reactions that lead to NOx adsorption. In a reducing environment, the catalysts activate reactions by which hydrocarbon reductants are converted to more active species, activate the water-gas shift reaction, which produces more active hydrogen from less active CO, and activate reactions by which adsorbed NOx is reduced and desorbed. In a typical operating protocol, a reducing environment will be created within the exhaust from time-to-time to regenerate (denitrate) the LNT.
A LNT can produce ammonia during denitration. Accordingly, it has been proposed to combine LNT and ammonia-SCR catalysts into one system. Ammonia produced by the LNT during regeneration is captured by the SCR catalyst for subsequent use in reducing NOx, thereby improving conversion efficiency over a stand-alone LNT with no increase in fuel penalty or precious metal usage. U.S. Pat. No. 6,732,507 describes such a system. U.S. Pat. Pub. No. 2004/0076565 describes such a system wherein LNT and SCR components are interleaved or co-disbursed over one substrate.
The present invention concerns the use of catalysts in LNT-based medium and heavy duty diesel truck exhaust aftertreatment systems. The catalyst requirements for LNT based-systems treating lean burn engine exhaust or diesel automotive exhaust are generally less stringent than those for treating medium and heavy duty diesel truck exhaust. Diesel exhaust is cooler than gasoline engine exhaust. Exhaust from larger diesel engines is cooler than exhaust from smaller diesel engine. At lower temperatures, reactions are generally slower and require more catalyst.
The LNT catalyst requirement depends on whether exhaust valves are used. Exhaust valves can be used to cut exhaust flow to an LNT during regeneration, rerouting the bulk or all of the exhaust flow until regeneration is complete. The precious metal catalyst requirements of LNT systems with valves may be less than those of otherwise comparable systems without valves. The valves permit the residence time of the reducing agent in the LNT during regeneration to be greatly increased. Also, the valves allow the environment within the LNT during regeneration to be easily controlled, Exhaust valves, however, are prone to failure and the present invention focuses on systems that do not require valves.
When valves are not used, regeneration involves eliminating most of the oxygen from the exhaust. Eliminating most of the oxygen from diesel engine exhaust generally involves injecting a reducing agent into the exhaust. The reducing agent reacts with oxygen and substantially consumes it. The reactions between reductant and oxygen can take place in the LNT, but it is generally preferred for the reactions to occur in a catalyst upstream of the LNT, whereby the heat of reaction does not cause large temperature increases within the LNT at every regeneration.
WO 2004/090296 describes a diesel automotive exhaust treatment system with a fuel reformer configured within an exhaust line upstream from LNT and SCR catalysts. The reformer has a high thermal mass. The reformer uses Pt and Rh to produce syn gas from diesel fuel at exhaust gas temperatures. For the reformer to be operative at exhaust gas temperatures, a relatively large amount of catalyst must be used.
U.S. Pat. Pub. No. 2004/0050037 (hereinafter “the '037 publication”) describes a different type of fuel reformer placed in the exhaust line upstream from an LNT. The reformer includes both oxidation and steam reforming catalysts. The reformer both removes excess oxygen and converts the diesel fuel reductant into more reactive reformate. Pt and/or Pd serves as the oxidation catalyst. Rh serves as the reforming catalyst.
The inline reformer of the '037 publication is designed to be rapidly heated and to then catalyze steam reforming. Temperatures from about 500 to about 700° C. are said to be required for effective reformate production by this reformer. These temperatures are substantially higher than typical diesel exhaust temperatures. The reformer is heated by injecting fuel at a rate that leaves the exhaust lean, whereby the injected fuel combusts to generate heat. After warm up, the fuel injection rate is increased to provide a rich exhaust.
The industry has found an apparent requirement for rhodium for valveless LNT-based diesel exhaust aftertreatment systems for trucks configured for regeneration using diesel fuel injected into the exhaust line. Without rhodium, NOx conversion efficiencies during LNT regenerations within the lower extent of the exhaust temperature range have been found to be unacceptably low. The rhodium is needed to produce more reactive substances from CO and large hydrocarbon molecules. As a result, rhodium prices have become very high.
In spite of advances, there continues to be a long felt need for an affordable and reliable exhaust treatment system that is durable, has a manageable operating cost (including fuel penalty), and is practical for reducing NOx emissions from diesel engines to a satisfactory extent in the sense of meeting U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations